The investment casting process is used to create metal components, e.g. turbine blades, by pouring molten metal into a ceramic shell of the desired final shape and subsequently removing the ceramic shell.
The process is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using a wax pattern die into which molten wax is injected. The wax pattern is then dipped in ceramic slurry to create a ceramic shell on the wax pattern. The wax is removed and the shell fired to harden it. The resulting ceramic shell has an open cavity of the size and shape of the final component into which the metal can be poured. The ceramic shell is subsequently removed, either physically or chemically.
It is necessary to produce single crystal components for use in certain environments. For example, a turbine blade must be highly resistant to creep at high temperatures and so a single crystal with its absence of grain boundaries is the preferred structure.
In order to achieve such a single crystal structure, a technique called directional solidification is used. This involves use of a spiral, “pig-tail” selector to initiate single crystal or grain growth followed by subsequent withdrawal of the mould (e.g. the ceramic shell) containing the cooling cast component from a heating zone into a cooling zone (having a chill plate) within a vacuum furnace at a predetermined rate such that the temperature gradient within the cast component is closely controlled. Generally, the “mushy zone” at the interface between the molten and solid metal in the cooling casting must be a calm (stagnant) interface with a flat temperature gradient in order to achieve a single crystal structure with no discontinuities. This is difficult to achieve in a component having a varying cross-sectional area along its profile because the heat radiated from the component will vary along its profile and convective instabilities will occur in the molten metal. As a result, an unacceptable proportion of cast components are rejected as a result of defects (e.g. freckling or secondary grains) formed during casting.
It is known from EP1452251-A1 to provide a horizontally oriented, displaced radiation deflector element in a mould for component casting. The radiation deflector element acts to reduce heat transfer between downward facing surfaces of the cast component (above the portions having a reduced cross sectional area) and the chill plate in the cooling zone of the furnace during directional solidification by acting as a barrier between the heating and cooling zones and reflecting heat back into the downward facing surfaces in the heating zone. This helps control (increase) the temperature gradient across the “mushy zone” which, in turn, helps reduce defects in the single crystal component by reducing buoyancy-induced defects such as freckling, sliver grains or high angle grain boundaries.
It is a preferred aim of the present invention to further control (increase) the temperature gradient at the liquid/solid metal interface (“mushy zone”) upon cooling of cast component having a portion with a reduced cross-sectional area e.g. a tang portion of a turbine blade.